A number of offset printing plate precursors for directly producing printing plates have hitherto been proposed, and some of which have already been put to practical use. A system in which a photoreceptor comprising a conductive support having provided thereon a photoconductive layer mainly comprising photoconductive particles, e.g., zinc oxide, and a resin binder is subjected to an ordinary electrophotographic processing to form a highly lipophilic toner image thereon and the surface of the photoreceptor is then treated with an oil-desensitizing solution, referred to as an etching solution, to selectively render nonimage areas hydrophilic, to thereby obtain an offset printing plate, has been widely employed.
Requirements of offset printing plate precursors for obtaining satisfactory prints are such that: an original should be reproduced faithfully on the photoreceptor; the surface of a photoreceptor has an affinity for an oil-desensitizing solution so as to render nonimage areas sufficiently hydrophilic, while, at the same time, having water resistance; and that a photoconductive layer having an image formed thereon is not released during printing and is receptive to dampening water so that the nonimage areas hold the hydrophilic properties enough to be free from stains even on printing a large number of prints.
It is known that the above performance properties of printing plate precursors are influenced by the ratio of zinc oxide to resin binder in the photoconductive layer. For example, as the ratio of resin binder to zinc oxide particles becomes small, oildesensitization of the surface of the photoconductive layer is increased to reduce background stains, but, in turn, the internal cohesion of the photoconductive layer per se is weakened, resulting in reduction of printing durability due to insufficient mechanical strength. On the other hand, as the proportion of the resin binder increases, printing durability is improved, but background staining tends to become conspicuous. With respect to background staining, while it is a phenomenon associated with the degree of oil-desensitization achieved, it has been found that the oil-desensitization of the photoconductive layer surface depends not only on the zinc oxide/resin binder ratio in the photoconductive layer, but also depends greatly on the kind of the resin binder used.
Resin binders which have been conventionally known include silicone resins (see JP-B-34-6670, the term "JP-B" as used herein refers to an "examined Japanese patent publication"), styrene-butadiene resins (see JP-B-35-1950), alkyd resins, maleic acid resins, polyamides (see JP-B-35-11219), vinyl acetate resins (see JP-B-41-2425), vinyl acetate copolymer resins (see JP-B-41-2426), acrylic resins (see JP-B-35-11216), acrylic ester copolymer resins (see JP-B-35-11219, JP-B-36-8510 and JP-B-41-13946). However, electrophotographic light-sensitive materials using these known resins suffer from disadvantages, such as low charging characteristics of the photoconductive layer; poor quality of a reproduced image, particularly dot reproducibility or resolving power; low sensitivity to exposure; insufficient oil-desensitization attained by oil-desensitization for use as an offset master, which results in background stains on prints when used for offset printing; insufficient film strength of the light-sensitive layer, which causes release of the light-sensitive layer during offset printing, thus failing to obtain a large number of prints; susceptibility of image quality to influences of environment at the time of electrophotographic image formation, such as high temperature and high humidity.
For the particular use as an offset printing plate precursor, formation of background stains due to insufficient oil-desensitization presents a serious problem. In order to solve this problem, various resins have been proposed as binders for zinc oxide, including a resin having a molecular weight of from 1.8.times.10.sup.4 to 1.times.10.sup.5 and a glass transition point of from 10.degree. to 80.degree. C. obtained by copolymerizing a (meth)acrylate monomer and a copolymerizable monomer in the presence of fumaric acid in combination with a copolymer of a (meth)acrylate monomer and a copolymerizable monomer other than fumaric acid as disclosed in JP-B-50-31011; a terpolymer containing a (meth)acrylic ester unit having a substituent having a carboxylic group at least 7 atoms distant from the ester linkage as disclosed in JP-A-53-54027 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"); a tetra- or pentamer containing an acrylic acid unit and a hydroxyethyl (meth)acrylate unit as disclosed in JP-A-54-20735 and JP-A-57-202544; a terpolymer containing a (meth)acrylic ester unit having an alkyl group having from 6 to 12 carbon atoms as a substituent and a vinyl monomer containing a carboxylic acid group as disclosed in JP-A-58-68046.
Nevertheless, evaluations of these resins proposed for improving oil-desensitization revealed that none of them is fully satisfactory in terms of stain resistance and printing durability.
Resins having a functional group capable of forming a hydrophilic group on decomposition have been studied for use as binders. For example, resins having a functional group capable of forming a hydroxyl group on decomposition as disclosed in JP-A-62-195684, JP-A-62-41-2426), and JP-A-62-210476, and resins having a functional group capable of forming a carboxyl group on decomposition as disclosed in JP-A-62-21269, have been proposed.
These functional group-containing resins form a hydrophilic group upon hydrolysis or hydrogenolysis with an oil-desensitizing solution or dampening water used during printing. It has been reported that the use of these resins as binders in lithographic printing plate precursors can avoid various problems associated with use of resins containing a hydrophilic group, such as deterioration of surface smoothness and electrophotographic photographic characteristics, which seem ascribable to the strong interaction between the hydrophilic group and the surface of the photoconductive zinc oxide particles. That is, when the resin containing a hydrophilic group is used as a binder, the binder adheres strongly to the surface of zinc oxide, thereby causing adverse effects, since (1) the hydrophilic property of zinc oxide is deteriorated and, thus, background stains tend to be generated due to inherently strong oleophilic property of the binder, and (2) the mechanical strength of the film formed lowers, thereby reducing the printing durability of the resulting printing plate. It has also been expected that the hydrophilic properties of the nonimage areas attained by an oil-desensitizing solution can be enhanced by the hydrophilic group formed by decomposition of the resin so that a clear distinction can be made between the lipophilic image area and the hydrophilic nonimage area. Adhesion of a printing ink onto the nonimage areas during printing can thus be prevented, thereby making it possible to obtain a large number of prints having a clear image free from background stains.
However, the above-described functional group-containing resins capable of forming a hydrophilic group are still unsatisfactory in resistance to background stain and printing durability. In particular, the resin becomes water-soluble as its amount is increased for the purpose of further improving hydrophilic properties of the nonimage areas, thus impairing durability of the hydrophilic properties. Hence, there is a demand to develop a technique by which the hydrophilic properties of the nonimage areas can be assured while lasting long. Namely, it has been keenly demanded to establish a technique to improve hydrophilic properties. Specifically, hydrophilic properties can be retained or rather enhanced even if the proportion of the resin containing a hydrophilic group-forming functional group in the total resin binder is decreased, or a large number of clear prints can be obtained without suffering from background stains even under strict printing conditions resulting from an increase of a printing machine size or variation of printing pressure.